Government officials will be looking at broadcast and published comments as well as speeches and sermons made by the trio to followers.

"No decision on charges has been made yet. The CPS will be looking at it to see if any offences have been committed," she said.

The Tories have suggested the review has been sparked by politicians rather than by law officers.

But Tony Blair's official spokesman said: "It is not a case of telling anybody what they should be doing. It is about looking at the options we have."

Possible charges which will be considered include the common law offences of treason and incitement to treason.

Outdated?

Lord Carlile said he would be surprised if treason charges were used.

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"I don't think there is a lawyer still alive and working who has ever appeared in any part of a treason case," the Liberal Democrat peer told BBC News.

Treason usually applied to wars between nations, he said.

Lord Carlile argued existing laws could be used, such as charges of solicitation or incitement to murder, commonly seen in contract killing cases.

He said new laws had to be proportional, enforceable and understood by the general public and those who could fall foul of them.

The ex-leader of al-Muhahjiroun, Anjam Choudry, attacked the idea of treason charges.

"On the one hand the government says you have freedom of expression, but on the other it wants to backdate things that people have said so they could face criminal charges, which is a betrayal in itself," he told GMTV.